Tools of the kind generally defined above are usually used for parting, grooving and slit milling. In connection with such working operations, as well as in connection with other chip removing machining in the modern engineering industry, often very strict requirements are raised as regards the tolerances of the machined workpiece. This in turn requires that the insert, on the one hand, be mountable in a very precise, predetermined position in the recess of the insert holder (the so-called insert seat) and, on the other hand, be capable of maintaining this precise position regardless of external action. Thus, the insert should be fixed immovably in its predetermined position relative to the holder not only when it is subjected to ideal main forces that act centrally or tangentially upon the insert (i.e., parallel to the plane of the insert holder), but also in relation to secondary transverse forces that strive to displace the insert sideways relative to the recess or the insert seat. Also small displacements of the insert sideways may lead to unacceptable tolerance deviations of the finished product.
Other requirements, that are basically contradictory in relation to the requirements as regards safe and precise clamping of the insert in the insert holder, are present in connection with the manufacturing of both the insert and the insert holder. Considered only from the point of view of the manufacturer it is thus desirable with the most moderate tolerance requirements that are possible, especially from the background that the components be manufactured in different processes and from different materials that among themselves have different properties. Modern inserts are manufactured from hard materials, like cemented carbide or ceramics. The most outstanding property of such cutting bodies is that they have high hardness and high wear resistance, implying a long life also in connection with machining of comparatively tough materials, like metals. However, a property of the hard body that is less desirable in practice is its brittleness; something that among other things means that the body runs a risk of being cracked or split in connection with unfavorable loading. Even if optimal manufacturing conditions always are sought, the manufacturing result as regards tolerances may in practice vary most considerably. The insert holders are usually manufactured from steel, this being an essentially softer and tougher material than cemented carbide. Also when the insert holders are manufactured, the result in practice varies as regards the achieved tolerances.
In theory, absolute maximum fixing of the insert in the insert seat would be achieved if as large surfaces as possible of the insert and the insert seat respectively initially could be maintained firmly abutting each other. In connection with practical series production it is, however, not possible, for the reasons given above, to separately manufacture on one hand all the inserts and on the other hand all the insert holders with such high precision that cooperating profile surfaces in the interfaces between inserts and insert holders fit absolutely perfectly into each other. In practice, the contact between the inserts and insert holders in the individual interface is established in two or at most three points or contact spots spaced-apart cross-section-wise (if more than three contact spots are present so-called redundant positioning would arise, i.e., the insert would initially be unstable and could tilt between different positions).
A circumstance that per se is not directly unfavorable for the determination of the position of the insert is that the insert after a certain, often short time of operative load imparts a certain plastic deformation to the insert holder. That plastic deformation will cause the insert to "settle". However, said deformation must not become so accentuated that the insert is distanced laterally from a central, predetermined position. Neither must the relative movements between insert and insert holder, in connection with plastic deformation, become so large that the more brittle insert runs a risk of being cracked and/or split.